High-voltage-activated Ca²⁺ channels are essential for diverse biological processes. They are composed of four or five subunits, including α₁, α₂-δ, β and γ (ref. ). Their expression and function are critically dependent on the β-subunit, which transports α₁ to the surface membrane and regulates diverse channel properties^,,. It is believed^,,, that the β-subunit interacts with α₁ primarily through the β-interaction domain (BID), which binds directly to the α-interaction domain (AID) of α₁; however, the molecular mechanism of the α₁–β interaction is largely unclear. Here we report the crystal structures of the conserved core region of β₃, alone and in complex with AID, and of β₄ alone. The structures show that the β-subunit core contains two interacting domains: a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain. The AID binds to a hydrophobic groove in the GK domain through extensive interactions, conferring extremely high affinity between α₁ and β-subunits^,. The BID is essential both for the structural integrity of and for bridging the SH3 and GK domains, but it does not participate directly in binding α₁. The presence of multiple protein-interacting modules in the β-subunit opens a new dimension to its function as a multi-functional protein.